Yes, August is hot, but as B9erSteve points out, not 24-7. Mornings and evenings are pretty darn nice. Highs in the 80s are most common, and if a heat wave hits, it usually doesn't last too many days. Just go spend an August in WDW and you will find the weather in Anaheim just fine. The last week of August is also pretty good for crowds, because many schools in CA. start at that time.
June weather is typically very pleasant. I would think the crowds would actually be lighter in June than the first few weeks of August even with the school trips. (But late August, as pointed out, if pretty good for crowds) Tourists and families seem to prefer July and early August from my experience. Also, this year, some may be putting their trip off until after Space Mountain re-launches.